1. Field of the Invention
The present invention relates to a polymer electrolyte membrane and a membrane/electrode assembly for a polymer electrolyte fuel cell.
2. Discussion of Background
A polymer electrolyte fuel cell is, for example, a stack of a plurality of cells each comprising a membrane/electrode assembly sandwiched between two separators. The membrane/electrode assembly comprises an anode and a cathode each having a catalyst layer and a polymer electrolyte membrane disposed between the anode and the cathode, and the catalyst layer and the polymer electrolyte membrane contain an ion exchange resin.
As the reduction reaction of oxygen at the cathode of a polymer electrolyte fuel cell proceeds by means of hydrogen peroxide, hydrogen peroxide or peroxide radicals will form at the cathode. Further, to the anode, oxygen molecules pass through the polymer electrolyte membrane from the cathode, and accordingly hydrogen peroxide or peroxide radicals may form in some cases also at the anode. Hydrogen peroxide or peroxide radicals formed at the cathode or the anode are known to deteriorate the ion exchange resin contained in the polymer electrolyte membrane and the catalyst layer.
As a polymer electrolyte membrane for a polymer electrolyte fuel cell and a membrane/electrode assembly, having durability to hydrogen peroxide or peroxide radicals, the following have been proposed.
(1) A membrane/electrode assembly containing ions of a metal selected from cerium, titanium, manganese, silver and ytterbium (Patent Document 1).
(2) A membrane/electrode assembly having a peroxide decomposition catalyst containing element selected from platinum, palladium, iridium, carbon, silver, gold, rhodium, ruthenium, tin, silicon, titanium, zirconium, aluminum, hafnium, tantalum, niobium and cerium (Patent Document 2).
(3) A polymer electrolyte membrane having a part of protons in cation exchange groups of the ion exchange resin ion-exchanged by ions of an alkali metal selected from lithium, sodium, potassium, rubidium and cesium (Patent Document 3).
(4) A polymer electrolyte membrane having a part of protons in cation exchange groups of the ion exchange resin ion-exchanged by ions of a polyvalent metal selected from magnesium, calcium, strontium, barium, zinc, copper, aluminum, gallium, indium, yttrium, lanthanum, titanium, zirconium and hafnium (Patent Document 4).
(5) A polymer electrolyte membrane containing cerium ions (Patent Document 5).
(6) A polymer electrolyte membrane containing cerium ions or manganese ions (Patent Document 6).
However, durability to hydrogen peroxide or peroxide radicals of the polymer electrolyte membranes for a polymer electrolyte fuel cell and the membrane/electrode assemblies (1) to (6) is still insufficient. Therefore, further improvement in the durability is required for a polymer electrolyte membrane for a polymer electrolyte fuel cell and a membrane/electrode assembly.
Patent Document 1: JP-A-2006-164966
Patent Document 2: JP-A-2005-538508
Patent Document 3: WO2009/157894
Patent Document 4: JP-A-2004-018573
Patent Document 5: WO2005/124911
Patent Document 6: WO2007/013533